Cemented tungsten carbide materials are known in the industry. This material constitutes a composite material of tungsten carbide (WC) that is embedded in a cobalt matrix. (Cemented tungsten carbide materials are sometimes abbreviated as “WC—Co”.) Typical compositions of the cobalt metal ranges from 3 to 30 percent by weight, although other percentages of cobalt may be used. For background information regarding cemented tungsten carbide materials, the reader is invited to consult U.S. Patent Application Publication No. 2005/0276717 A1 (which publication is expressly incorporated herein by reference).
Cemented tungsten carbide materials have unique properties compared to steel, metal alloys, or ceramic materials. For example cemented tungsten carbide materials have higher hardness, wear resistance and strength, as compared to steel. However, cemented tungsten carbide materials have been known to more easily fracture than steel. In the industry, this property of the material is referred to as “fracture toughness.” Fracture toughness refers to the propensity of the material to chip or fracture during use as a result of the service under mechanical loading. Thus, cemented tungsten carbide materials have less fracture toughness than steel. When compared to ceramic materials, WC—Co materials have higher fracture toughness and have equivalent or better hardness and wear resistance than do the comparable ceramic materials. Because of these unique properties, many cemented tungsten carbide materials are used in a wide range of industrial applications including metal cutting tools, mining tools, oil and gas exploration tools, and many other applications requiring extreme wear resistance.
Many WC—Co materials have a relatively low fracture toughness which limits their effectiveness in some potential applications. This low fracture toughness means that the material has a propensity to chip or fracture during use. This chipping and fracturing is especially prevalent when the WC—Co material is used as a cutter on a rock drill bit. Chipping and fracturing are the leading causes of degradation or premature failure or cemented tungsten carbide tools.
Another type of material that is used for metal cutting tools, mining tools, oil and gas exploration tools, and other similar applications is polycrystalline diamond (“PCD” or “Cd”) materials. PCD materials are composed of diamond crystallite particles with a small percent of cobalt residing in the inter-particle spaces formed by the diamond crystallite particles. In many instances, PCD materials have many of the same advantageous properties that are generally associated with WC—Co materials. At the same time, many PCD materials suffer from many of the same deficiencies as WC—Co materials.
As noted above, one of the common modes of failure of tools made of WC—Co and/or PCD materials is that they tend to crack, chip, or fracture during use. Such cracking or failure of these materials is especially common when the tool become hot (as a result of the friction caused by contact with another surface). This cracking of the tool as a result of frictional heat is sometimes referred to as “heat checking.” (The cracks formed by frictional heat may also be referred to as “thermal fatigue cracks”). FIG. 1 illustrates a typical appearance of tool with thermal fatigue cracks. For example, the surface 10 of the tool 14 includes one or more cracks 12 formed by heat checking.
The formation of heat checking cracks will now be described. The tool (cutter) engages the surface of a rock formation or a work piece. For example, if the tool is a rock drill mining tool or a construction tool, the tool may engage the rock formation when it is being used to cut the rock formation. Likewise, the tool may be used with a work piece in applications such as metal removal or in the formation of engineered wear parts (e.g., abrasive nozzles and seal rings). When the tool engages the surface of the rock formation or work piece, the temperature at the contact point and the adjacent surface areas increases dramatically from ambient temperature. This increase in temperature is due to intense frictional heating.
For example, in the case of cutting inserts on a lathe, the peak temperature at the tool's cutting tip can reach temperatures above 1000° C. The temperature at the contact between a rock drilling cutter and rock formations can reach temperatures above 800° C. The actual peak temperature at the cutting edge of the tool is obviously a function of surface speed (i.e., the speed at which the tool contacts the surface), the depth of cut being performed, the load (or force) applied to the surface, and, more importantly, the thermal conductivity of the cutting tool material. The thermal conductivity of the tool refers to the ability of the material to dissipate heat by conducting the heat out of the cutting contact area into other areas of the tool. When the thermal conductivity of the tool is “low,” the temperature of the cutting area will be higher because the tool is not able to dissipate the heat away from the cutting area. Alternatively, if the thermal conductivity of the tool is “high,” the temperature of the cutting area will be lower because some of the heat associated with the cut will be dissipated to other areas of the tool.
In order to reduce the frictional heat that is imparted to the cutting area, most cutting and drilling operations will use coolant or a cooling system to cool the tool and/or the cutting area. Then, when a coolant or cooling system is used, the cutting tool may experience drastic and sudden changes in temperature, namely, sudden heating caused by the frictional contact and then sudden cooling caused by the cooling system. These sudden changes in temperature may cause the thermal fatigue and thermal cracking of the tool.
It should also be noted that cemented tungsten carbide, and polycrystalline diamond (PCD) are essentially composites of the primary phase WC (or polycrystalline diamond) and the matrix or binder material. (Generally, the matrix or binder material is cobalt). The hard particles of the primary phase (WC or Cd) have much lower coefficients of expansion than that of the matrix ductile metal matrix phase (e.g., Co). Accordingly, the high temperature associated with tool usage also creates cyclic residual stresses between the hard particles and matrix phase. Such cyclic residual stresses can be an additional cause of heat checking cracks (thermal fatigue) of the material.
Of course, heat-checking cracks in a tool surface can propagate and get larger over time. When such cracks exceed a “critical size,” the tool can no longer be safely used. At this time, the tool must be replaced and discarded, thereby increasing the overall costs of the cutting process.
Thus, there is a need in the industry for a new type of WC—Co or PCD material that may be used in industry as part of a cutting tool. In generally, this new type of WC—Co/PCD material should have high thermal conductivity such that it is capable of readily dissipating the heat caused by frictional contact with the cutting surface. Such heat dissipation would reduce the temperature at the cutting edge, thereby resulting is lower residual thermal stresses. Such dissipation of heat would also reduce the amount of heat cracking of the tool. Such new materials (as well as the tools that incorporate these materials) are disclosed herein.